I too use that
Casio watch, and I SWEAR by it. Basically what I do is set Interval 1 for 15 seconds - so the watch vibrates when there are 10 seconds remaining - and then I set Interval 2 for 10 seconds - so the watch vibrates when the 25-seconds has expired. I absolutely love this watch, thanks to whomever originally suggested it on this board.
You may need to look around to find it though, because I don't know if it's available anymore.